I store mine horizontally, fully supported on a wire metal shelf. Somewhere that is dry and gets a little air circulation. Make sure the flooring is fully supported so it does not sag.

Lay the cartons flat.
Under a bed or on a shelf inside the home, is best.
In the Garage, as long as it is off the concrete floor and it is not overly humid in there.

If the wood is stored under the bed -- is it ok to transfer the wood to a plastic under bed storage container?

Have anyone tried to store the extra wood planks in the attic? I'd like to hear your experience. I've read that the main concern is 'humditity' and not temperate rage. But attic, again, is not a climate controlled environemnt either.